1. Field of the Invention
This invention relates to a microcapsule type toner to be used for electrophotography, electrostatic printing, magnetic recording and the like, and a process for producing the microcapsule toner.
2. Description of the Prior Art
In the prior art, as the toner for electrostatography, electrostatic printing or magnetic recording, use has been made primarily of resinous particles prepared by dispersing in a resin a dye or a pigment, and a magnetic material, if desired, followed by kneading and crushing into fine particles of about 5 to 30.mu..
A toner for these processes is required to have a variety of performances, including developing characteristic, fixing characteristic, durability, stability, resistance to environmental conditions and others, and a single material can hardly satisfy all of these various performances. Accordingly, there has been proposed a so called microcapsule toner, in which the function related primarily to the surface of the toner particles such as developing characteristic is separated from the function related primarily to the bulk of the toner such as fixing characteristic, namely, one comprising a core material with good fixing characteristic enclosed within a material excellent in developing characteristic. Particularly, in recent years, a large number of machines utilizing the pressure fixing system have been reported, in which, in place of thermal fixing system, fixing is performed by pressing down a toner against a fixing substrate (mostly, plain paper). This pressure fixing system has a number of advantages. thus no or little, if any, heat source is required accompanying danger of fire as well as the risk of scorching of copied sheets, and the device can also be simplified. Also, no waiting time before heating of a fixer is necessary and thus adaptability for high speed copying is high.
As an example of such a microcapsule toner, there is a capsule toner containing a soft material as the core as discloed in Japanese Patent Publication No. 8104/1979 or a capsule toner containing a soft resin solution core as disclosed in Japanese Laid-open patent application No. 132838/1976.
However, microcapsule toners proposed heretofore still involve a large number of problems and are far from satisfactory in practical application. This may be partly because of the fact that a material suitable as the toner material is not necessarily suitable as the material for microcapsule, while it is difficult to impart suitable developing characteristics for a toner, particularly charge controlling characteristic, to a material for microcapsule, particularly a material constituting walls.
In an encapsulation process presently practiced frequently in the art, a solid core material is dispersed in a solution of a wall material for enclosing the core material therein, and the solvent is removed by heating or other means thereby to precipitate the wall material around the core material. This process has the advantage of availability of materials with desired characteristics in combination such as, for example, a material with excellent fixing characteristic and a material with excellent developing characteristic, but the combination of available materials is limited due to the use of a solvent. Also, even if one of the limited combinations is adopted, the core material cannot completely be insoluble in the solvent employed. Particularly, it is difficult to completely prevent a low molecular weight component, which is to be added intentionally for improvement of fixing characteristic, from being dissolved out into the solvent. As the result, there are involved problems that such as component dissolved out may interfere with adhesion of the wall material on the surface of the core material, and also that no sufficient functional separation is attained due to entrainment of the core material into the wall material, which have adverse effects on developing characteristic and durability.
Further, even a microcapsule toner having overcome the drawbacks as mentioned above still has a problem of peel-off of the wall material caused by the shocks during developing operations, and under the present situation, there remain a large number of problems to be solved before practical application of the microcapsule toner such as completeness in coating, toughness of coating, and also pressure fixability as the basic characteristic. More specifically, there has been obtained no practical pressure fixing toner, which has the characteristics of excellent pressure fixability without off-set phenomenon onto the pressure rollers, stable developing performance and fixing performance for repeated usage without adhesion to carrier, metal sleeve or the surface of a photosensitive member as well as good storage stability without agglomeration or caking during storage. In particular, there remains a problem with respect to the pressure fixing characteristics concerning the fixing characteristic onto plain paper. Furthermore, in the capsule toner as described above, the adhesive force between the core material and the wall material is weak, whereby peel-off of the wall may partly occur, thus frequently causing problems such as changes in image quality and image density due to increased triboelectric charges in continuous copying tests or fusion of the wall material onto a developing sleeve or the surface of a photosensitive member.
In addition to the above mentioned prior art, the research group to which we belong has developed a microcapsule toner having an outer shell layer comprising a cyclized rubber (U.S. Pat. No. 4,265,994). The microcapsule toner can take a double-layered wall structure having an insulating resin layer overlying the cyclized rubber layer. This double-wall microcapsule toner has, however, sometimes caused peeling-off of the insulating layer to cause contamination of equipments for development and fixation and result in somewhat poor quality of images after a long term of continuous copying operation.